text:
Would a pilot know that one of their crew is armed?
The Federal Flight Deck Officer page on Wikipedia says this:

Under the FFDO program, flight crew members are authorized to use firearms. A flight crew member may be a pilot, flight engineer or navigator assigned to the flight.

To me, it seems like this would be crucial information for the PIC to know, if their flight engineer (for example) was armed; but on the flip-side of this, the engineer might want to keep that to himself if he's with a crew he hasn't flown with before.
Is there a guideline on whether an FFDO should inform the crew that he's armed?

It will become rather apparent when you see the crew member unlock, load and holster their weapon.

@casey It has to be holstered in a particular place? Otherwise the engineer might want to keep that to himself

You should clarify whether you are asking about FFDO or "not commercial aviation" as FFDO only exist at airlines.

@casey Ah I see! Let it be commercial then. I'll edit momentarily.

It has to be briefed as different door procedures will be in effect. Ill figure out what I can or cant say and post an A later.

This question can't legally be answered by anybody who actually knows the answer and should probably be closed.  I can't find a suitable reason under the close link though.

@casey Did you manage to figure this out?

Seriously, we can't answer your question.  I wouldn't even have said as much as casey already did.  It's nothing personal.

@Ralgha Then post an answer with the reasoning for that, citing a source; this is the first I've heard of it. Btw, you can choose Close > Off-topic > Other if you think the question deserves closing; though I'd respectfully disagree that it's off-topic.

It's not off-topic, which is why I didn't vote to close it.  It simply can't be answered.

@DannyBeckett SSI and I won't comment further than I already have.

@casey Hey, no worries! It was just cos you mentioned...

The answer to this question is protected as Sensitive Security Information (SSI) and can't legally be disclosed here.  Anyone who does answer the question either doesn't know what they are talking about, or faces steep legal consequences.
Many, even most, questions relating to airline security (in the US at least ) are similarly handicapped.


text:
How can Ryanair offer flights for a penny?
Of course, we all know that you're not exactly treated like a rockstar on Ryanair, and their CEO wants to charge us £1 to use the toilet; but still, how can they offer flights for a penny?!
Looking at flights with budget airlines, they usually end up costing more, with taxes etc added on, but I've seen Ryanair offering flights for literally a penny, all in.

Fees. Lots and lots of fees. They'd levy a breathable-air surcharge if they thought they could get away with it.

Even if they paid me to take a flight with them, I'd still decline. The answer btw, it's just a loss leading sales gimick

it looks like some of the price is paid just by all those adds on the page

Ye Gods - I know how they can do it! All the money they're obviously NOT spending on a web designer! :)

@egid In the UK, the advertised price must include all taxes and non-optional fees.

The hilariously musical (if slightly NSFW) answer is "They can't."

Have you ever ridden in the cargo hold? It's not very comfortable...

Besides @Sebass van Boxel's answer, I'd like to add the following:
I was talking to a Fastjet pilot (ex easyJet) and he was telling me that obviously not all seats are available for a cheap price.
If you book early you will get a special price but try booking a day before and you will see.
As has been said, a lot of money is made on all the extra services such as luggage, food, etc.
From what he told me, the profit on each flight for easyJet is minimum \$400 per flight at 96% full — which is not much, but consider that easyJet has something like 1,000 flights a day and that means \$400,000/day profit. Not bad...
That how the money is made.

especially that: per flight only 1-2 seats are offered at that special price, every other seat is more expensive.

Airlines in general are experts in price differentiation. Tickets for any particular flight of any airline were probably purchased at dozens of different rates. At one penny it's a loss leader and indeed there would be just a few seats in each flight offered for such low price and only on few lines, so people are attracted to them with the advertisement but end up buying different ticket for price that is still low but profitable.

When I worked for Continental Airlines 20yrs ago, we were told the flight was paid by the time first class was filled and anything after that was mostly pure profit.

One of the reasons Ryanair has some benefits on other airlines are the subsidies they get from airports.  Basically taxpayers are paying a part of your ticket (See this businessweek article).
Next to that they fly on relatively cheap times and small airports which means they have to pay less to an airport.
Lastly; as you know that charge fees for everything. Whether you want to bring an extra bag, something to drink or like to go to the toilet in the feature, you'll have to pay for it.
I wonder whether they can make money when people don't want to make use of the 'extra's' they offer. I don't think so.
Maybe someone can do the math and calculate what an average Ryanair flight costs (See this answer) and how much money they get from only the tickets.

pretty much every airline gets subsidies in some places and has to pay fees in others (or even in the same location). That's not a factor.

@jwenting Good point. Though Air France-KLM sues them for the illigal subsidies they get, proofs they are - trying - to get more subsidies than other airlines. http://www.digitallook.com/dl/news/story/3332999/Air_France-KLM_sues_Ryanair_over_alleged_subsidies.html

I'm sure AF/KLM could be countersued for the same, especially for operations in Paris... And French government interference providing AF landing rights in exchange for discounts on Airbus aircraft is also well documented.

@jwenting Can you provide an article or source about that?

The marketing term is "loss leader" - you lose money on the advertised product but make more money on other things. In this case fees and other things the customer might buy ( checked bags, water ) after we get them in the door (website).
Your grocery store does this weekly. For example: 2 litre Coca-cola $0.49 deals.

Under European laws they can't advertise price without fees. It's other things the customer might buy and more importantly all the customers that come to their site because of the advertisement, but end up buying different, more expensive ticket (while they only provide few at the lowest price).

@JanHudec then RyanAir is eating the airport fees and taxes themselves and genuinely losing money on the 2 available seats at that fare. They have done the math and it eventually works out for them.

The real answer has not been given yet, it boils down to marketing. Offering flights for a penny sounds to good to be true for most people. It makes them curious and draws them to the Ryanair website. That is exactly what Ryanair wants, it is free publicity. Then for given flights there are a set of seats available for that price. It will be on the days that most people can't fly for a leasurly reason thus during weekdays or on flights that draw not that many customers.
But since people have been drawn to the website they stick around and try to find a cheap deal. Ryanair knows this and the whole website is set to encourage you to buy a flight, they state how many seats are available for the discounted price, encouraging you to keep searching for a cheap deal. In the end most people will settle for a reasonable cheap flight to a destination which was not there first choice but the flight was cheap so they bought it. ;)
As of now they state that they have 500.000 seats available for discounted price of 25% off, they have roughly 2000 departures a day, with 189 seats per plane this works out to 378.000 seats departing on a single day. the campaign runs for a month so that results in 11.3 million seats. In the end they have put 4.4% of there seats for that month on discount. Not that many, that is roughly 8 seats per aircraft or little over 1 row of seats. In the end it is just a very cheap marketing strategy!
Then the part of the question, How can they offer them so cheap? Willingness to pay is the key, last minute seats are expensive, since you obviously need to go somewhere in a hurry, this means that you are willing to spend more on a ticket. These customers pay for the cheaper tickets bought way in advance buy others. Routes with a high demand also allow the airline to ask more for a seat.

"how can they offer flights for a penny?!"

Do some research on the topic of Yield Management (hint: only a small proportion of people on any given flight will pay "a penny").
All airlines work on yield management for their pricing.  Just the LCCs (Low Cost Carriers) have taken it to a whole other level.
Its a fascinating topic. But incredibly complex, especially in the "cutting-edge" form adopted and embraced by the LCCs.
P.S. Revenue Management is another big topic to look into in relation to airline commercials, again its an area all airlines do but again turned into an artform by the LCCs. Another one to investigate once you're done reading up on Yield Management !

As mentioned above, there are many (hidden) fees.
Plus, Ryanair uses small airports with small amount of movements a day, which gives the advantage to Ryanair when creating contracts with these airports. Basically, airport has 15 minutes to refuel, check, clean and service aircraft after landing, and the aircraft has to be ready to leave within this 15 minutes period. If the airport doesn't meet these requirements, the airport has to pay a huge fee (can't remember how much exactly, but at least close to two thousands Euro, maybe more).

Other than the fees for delays, which would be good to have a reference for, this doesn't add anything to other answers.

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Where does my pee go when I flush at 35,000 ft?
One thing I guess a lot of people wonder is: what happens you flush the toilet in an aeroplane?
Does it fall out the belly of the plane and disintegrate? :)

A kiwi woman complains that "aeroplanes are taking a s**t on her driveway", beneath the flight path. Is there any way this might actually happen?

I think it is stored and then removed from the tanks at the next stop..

Obviously, not thrown on people! but collected and replaced after landing

See http://gizmodo.com/5953877/what-happens-when-you-flush-a-toilet-on-an-airplane for a good article.

I can speak for the most common commercial models (the 7x7s, A3xxs and the like) via an engineer friend.  All of these models flush into a tank which is emptied on the turnaround.
For Vulcans, I can speak with first hand experience.  Each crew member had a stainless steel "pee tube", a funnel, which fed into a rubber bladder.  They are emptied and cleaned during the after flight inspection.
I was changing a radio under the rear crew table where the electronics, navigator and radar operator sat.  This involved squeezing down behind the seats to lie under the table, disconnecting and unfastening the radio, hauling onto your chest then pushing it up onto one of the seats and reversing the process to get out from under the table.
As I pushed the radio up onto the seat, I knocked one of the pee tubes and bladders out of it's clip holder which then emptied the contents onto me and the radio.
The bladder had not been emptied - and the aircraft had been in the hangar for 3 weeks!
Of course, it was my job to clean up the mess - lots of french chalk as urine is very corrosive to airframes.
The radio also needed writing up so I tagged it as "suspect urine ingress".
The same radio came back to me with a tag attached saying (and this might work only for non-American English speakers) - "are you taking the p*ss?".
Yeuch.

Yes in America we use piss as a slang for urine as well. Nice answer BTW, I love answers that include some personal experience. (Even if it is a stinky situation).

As opposed to taking a pisser..

On newer airliners it is fed into a septic tank. Decades ago, it was simply dropped overboard. Before the introduction of vacuum flush toilets, water flushing was used which involved big tanks which could leak their content overboard.
The Boeing 727 had at least two cases where the right engine was lost as the ultimate consequence of this malfunction. Since the lavatory piping was on the right side, ice which had built up there could break loose and be ingested by the engine behind. If the ice block was large enough, it could damage the fan, and the resulting imbalance could shake the engine off. Boeing engineers designed the engine mount with a rated break point so hefty engine vibrations would not damage the rest of the plane.
Glider pilots have all kinds of means to lighten themselves when nature calls. The product of this is then thrown overboard - that is what the small sliding window on the left side of a glider canopy is for. A friend of mine uses plastic bags and once forgot to take one with him in a competition flight. He realized his omission just before he was aero-towed on his way and asked bystanders for help. Someone handed him his plastic bag which had been used to hold apples - and was perforated. My friend realized this detail too late, but very soon after he began using the bag.

It is stored and then disposed of when plane touches down.
From Yahoo! Answers and Wikipedia: Aircraft lavatory.

there's also the ubiquitous MythBusters and their Blue Ice episode

@DavidRicherby Although the guy that wrote the answer did claim to have worked as a 'poo technician' and considers himself an "expert in poo and pee on aircraft" :P

It's disposed of when it touches down!  Wow, that must make the runways slippy ;)

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What to do if the captain smells like alcohol?
Suppose you're the co-pilot on a flight and during pre-flight, talking to the captain, you smell alcohol.
Although you didn't see him drinking and the smell isn't strong, what does regulation stipulate you should do?

Meta discussion of this question.

This is an interesting bulletin, but it only really concerns BAA Security suspecting a crew member of drinking: http://afausairways.org/Eline/mar28a_13.htm

Here's something from the NY Times' archive from '85: http://www.nytimes.com/1985/04/17/us/alcohol-rule-is-set-for-all-pilots.html

When I originally saw this question, I refrained from commenting on it because I realize that my views on the subject are old-school so to speak, and I'm probably in the minority. The question should not be whether or not an individual has a certain level of some substance in his blood stream. The question should be whether or not he/she is impaired. I well remember flying night freight on a contract out of Madrid and having to put up with F.O.s who were technically legal but hung over to an extreme degree. They were Americans who weren't used Spanish beer.

Now for the part that may get me some flak. No problem, though, as I'm 75 and my flying days are long over. More than once I detected the slight usage of alcohol by a crew member. In every case I chose not to do anything because I didn't feel that the safety of the flight would be compromised. That said, I once ordered a ground crew member to quit working on the load because I detected what I believed to be an excessive amount of alcohol on his breath, and I worried about him not properly getting all the locks set on our load.

@Terry Thanks for sharing your experiences; it's interesting! You should consider converting your comments into an answer.

Unfortunately, at least in the US, the regulations don't stipulate that you do anything. Airline policy is a different matter, and one which I cannot speak to, as it will be different in every case.
I can provide some background and my own take on this, but it's hard to have one exact answer (what if you're wrong?).
FAA regulations rely on pilots self-regulating. If a pilot drinks within an 8 hour (or, as is often policy for airlines, 12 hour) window prior to the flight, they are required to remove themselves from that flight. If they do not take this action, somebody else must. Often, this person is the First Officer, a junior pilot who has years of flying with pilots who might come to view them as untrustworthy. Perhaps this can help explain why it's such a difficult action to take.

This is a difficult topic, and one which regularly comes up during airline flight training. That said, the courses I took that discussed this scenario essentially landed on a couple of steps (this was years ago, this is probably a bit rusty):

Confront the pilot. Ask them if they've been drinking in the last X hours. If they claim they have not been, or they push back:
Attempt to explain to them the situation they're putting their career (and you) in;
Try to get them to remove themselves from the flight and contact a union representative.

Beyond that, I don't want to make suggestions. Getting them to talk to the union they are a member of is probably a wise idea, as they can offer possible solutions or arrange treatment if the pilot has alcohol abuse problems (which is not a huge stretch, if they're showing up to work reeking of booze). AOPA Flight Training has a blog post that discusses some similar scenarios.

"...the regulations don't stipulate that you do anything."  Are you sure? "(b) Except in an emergency, no pilot of a civil aircraft may allow a person who appears to be intoxicated or who demonstrates by manner or physical indications that the individual is under the influence of drugs (except a medical patient under proper care) to be carried in that aircraft."  The co-pilot is still a pilot.

You can make them call in sick, but you are not required to report them.

This is clear cut. You are required to wait 8 hours after taking a drink of alcohol before you fly a plane. According to FAA regulations:

(a) No person may act or attempt to act as a crewmember of a civil
  aircraft—

(1) Within 8 hours after the consumption of any alcoholic beverage;
(2) While under the influence of alcohol;
(3) While using any drug that affects the person's faculties in any
    way contrary to safety; or
(4) While having an alcohol concentration of 0.04 or greater in a
    blood or breath specimen. Alcohol concentration means grams of alcohol
    per deciliter of blood or grams of alcohol per 210 liters of breath.

(b) Except in an emergency, no pilot of a civil aircraft may allow a
  person who appears to be intoxicated or who demonstrates by manner or
  physical indications that the individual is under the influence of
  drugs (except a medical patient under proper care) to be carried in
  that aircraft.
(c) A crewmember shall do the following:

(1) On request of a law enforcement officer, submit to a test to
    indicate the alcohol concentration in the blood or breath, when—

(i) The law enforcement officer is authorized under State or local law
      to conduct the test or to have the test conducted; and
(ii) The law enforcement officer is requesting submission to the test
      to investigate a suspected violation of State or local law governing
      the same or substantially similar conduct prohibited by paragraph
      (a)(1), (a)(2), or (a)(4) of this section.

While technically correct, this is not answering the question. The question relates to what a crew member should do if they suspect another crew member is intoxicated.

An airline's operations manual may impose stricter regulations. At the two Part 121 carriers I worked for, it was 12 hours.


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Is it legal to smoke in your own plane?
I was just curious, in the past, smoking on planes was permitted.
What is the law surrounding smoking in your own private plane?

I am reminded of United States Air Force general Curtis Emerson LeMay, who was equally famous for his overbearing command style and his ever-present cigar.  He smoked it on his own transport plane, on and around air force bombers in the Pacific theatre, on the flight line, while observing fueling and arming operations -- everywhere.  He was such a precipitous and powerful leader that his men believed no aircraft or fuel cell would ever dare to explode in his presence.

@A.I.Breveleri You reminded me of the scene in the 1955 movie "Strategic Air Command" in which LeMay (though not using his name) was played by Frank Lovejoy. He gets off his personal airplane onto the ramp next to, as I remember, a brand new B-47. I want to say the B-47 was being fueled, but I'm not sure of that. Somebody says to the General's aide something like, "He shouldn't smoke here. The airplane might blow up." The aide looks at the man and says, simply, "It wouldn't dare."

The short answer to "Can I smoke in my own private aircraft?" is "Yes, usually".
As Promised, I looked up the relevant FAR (23.853 - Passenger & Crew Compartment Interiors).
The actual rule is long and verbose, but pretty common sense. The important regulatory bits for most personal aircraft are just two points though. You must:

Have "an adequate number" of self-contained ash trays
You don't want flicked ash or a snubbed cigarette to start an in-flight fire.
Have "No Smoking" signs you can illuminate, if the passenger/crew compartments are separated
This doesn't apply to most of us since our "personal" airplanes are light singles, but if you've got a fancy twin with a curtain or door between you and the passengers you'll need those signs.

For "commuter category" aircraft there are even more requirements. Those requirements don't apply to most "personal" planes, so even though it's probably a good idea to meet some of those requirements if you're going to allow smoking in your personal aircraft they're not legally required.

All that said, while it's likely legal (unless you've removed your ashtrays or your plane didn't come with them) common sense generally dictates you don't want to smoke in your plane - It's bad for the vacuum-driven instruments' filters, will require even more attention to cleaning the inside of your windows, and a loose cigarette in turbulence or a bunch of ashes blown about by an open air vent can be a pretty substantial distraction.

I'm not sure where Light Sport aircraft fall on this issue - new LSAs are generally not covered/certificated under FAR 23. I suspect like many other LSA things it's "Does the manufacturer explicitly permit/prohibit it in the POH or via placards?"

Did you manage to find anything about the dependency on what materials are used in the plane?

@DannyBeckett I wasn't actually - I may have been remembering something specific to part 121/135 or transport-category aircraft. It was something about the difference between "flame-resistant" and "self-extinguishing" materials (where the former required a "No Smoking" placard, and it was OK to allow smoking if the cabin was outfitted with the latter). At some point I'll have to go on an archeological dig through the FARs to see if I can find it again.

What about airplanes certified under Part 25 instead of Part 23?

I don't know that any Part 25 (transport) aircraft would qualify as "personal", though maybe Warren Buffet wants his own 787 - you know, to draw the Berkshire-Hathaway logo with. The requirements seem similar to Part 23 - It's 25.853. Some areas of 23.853 also specify that they apply to "transport category aircraft" as is the way of federal law's confusing and tangled web.

Not to mention bad for the health of the pilot, and with the reduced oxygen at altitude healthy lungs are quite important to reducing hypoxia. You didn't seem to be endorsing it anyway but I just thought I might add that in there.

@voretaq7 Donald Trump has his own 757-200, N757AF.


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I just saw a plane drop off online radar; should I do anything about it?
I was watching Flight Radar 24, positioned near my house, to see what planes I could spot, and noticed a Cessna 550's transponder just dropped off @ 5,325 ft in a high-speed descent!
Is there something I should do about it? Or do I assume somebody in ATC has seen it?
Is there anything anyone should do in similar circumstances?

This is almost a dupe of this question

@Pondlife Same fish, different pond -- I think this one can stand on its own as a reminder to people that they don't need to call emergency services if they see a flight disappear from FlightAware.

@voretaq7 Yes, I agree; I find it's often interesting/useful to link similar-but-different questions. The different perspectives and details are often good to know.

In addition to the answers below, have a look at http://www.flightradar24.com/faq - "I was following an airplane when it seemed to have disappeared, why?"

Really there's nothing you should do in cases like these.
Flightradar24, FlightAware, and similar services should not be used for flight safety purposes, and most of them specifically state so in their terms of service (such as sections 12 and 14 of Flightradar24's terms and conditions). Their sources may go down for whatever reason -- that does not mean that the plane has crashed.
These services can only track what ATC controllers can (for the most part), so it's more likely that if there is a real issue, that they'd notice without the general public making a fuss.

In this specific case the flight was being tracked using MLAT (designated by the radar type T-MLAT, which is shown in your animated image file). MLAT flight track accuracy relies on the accuracy and number of the receivers in the area, and according to Flightradar24's website, MLAT generally only works above 10000 feet:

MLAT
In some regions with coverage from several FR24-receivers we also
  calculate positions of aircraft with the help of Multilateration
  (MLAT), by using a method known as Time Difference of Arrival (TDOA).
  By measuring the difference in time to receive the signal from
  aircraft with an older ModeS-transponder, it's possible to calculate
  the position of these aircraft. Four FR24-receivers or more, receiving
  signals from the same aircraft, are needed to make MLAT work. That
  means that MLAT coverage can only be achieved above about 10000-20000
  feet as the probability that signal can be received by four or more
  receivers increases with increased altitude.
MLAT coverage is today limited to some parts of Europe and North
  America, but expanding fast.

So as the aircraft descends below 10,000 feet there is an increasing chance that you will lose it on Flightradar24's tracking system.

I would add that loss of RADAR contact does not mean that they are not still in contact with ATC

one more thing: those websites/systems are always delayed by quite a lot, and have a low sample rate, so by the time you see something drop away people will already be on it if there's anything wrong.

Nothing you should do.
Flight Radar 24 does not have coverage down to ground level in all areas. ATC has better surveillance and will be aware of the aircraft. Probably it's just in a fast descent, the Citation II / Bravo is quite a nice toy.


text:
What's the difference between True vs Magnetic headings?
What is the difference between a true and a magnetic heading?
Why are/were magnetic headings used in aviation?

The "heading" refers to the direction an aircraft is pointing. For a Magnetic Heading, this is in relation to Magnetic North. For a True Heading, this is in relation to True North.
True North is directly over the earth's axis.
Magnetic North is somewhere over Canada, moving towards Russia.

To get the Magnetic Heading, you just read it off the magnetic compass.
To get the True Heading, you need to first read the magnetic compass, then either add an Easterly, or subtract a Westerly, magnetic variation; based upon the isogonic lines on your sectional (the purple dashed lines labeled 5°W, 3°E, etc).
Example 1: Magnetic Heading 177 w/ 3 degrees East Magnetic deviation = true course 180.
Example 2: Magnetic Heading 177 w/ 3 degrees West Magnetic deviation = true course 174.

Because of this, in the past, magnetic headings were used because a simple compass could be used. Finding reliable true headings was difficult until the era of things like the gyrocompass (patented in 1906 (Germany) and 1908 (USA)) and more recently, GPS.

You also need to take into account the compass deviation table which (hopefully) compensates for built-in magnetic influences.

Here's a question and answer what happens when you follow a constant magnetic heading due to compass deviation:http://aviation.stackexchange.com/questions/13248/klax-hdg-090%c2%b0-cross-the-atlantic-where-are-we-now/13249#13249

Note that gyrocompass only works when stationary or moving slowly, so while it existed in the early days of aviation, it couldn't be used on aircraft. Only with introduction of inertial navigation systems aircraft got gyroscopes good enough to align before departure and then maintain the true north with useful precision during the flight.

Also GPS alone can't tell you true heading, because it can only calculate position and from it true track, but that differs from heading by (unknown) wind. What it can be used for is improving precision of INS.

True North is the North Pole. The maps used for navigating are oriented to the North Pole. A pilot can measure the direction between two points to create a 'track' or 'course' to fly in degrees true.
Magnetic North is where the compass points - which is not actually the North Pole. Magnetic North is currently over northern Canada.
The angular difference between True and Magnetic north is called Magnetic Variation. This number varies depending on where you are on the earth.
Magnetic Variation is the bugbear of many student pilots. Why is it important? Well if your track to fly (ascertained from a map) is 360 degrees true in relation to the North Pole, you should be flying right to the North Pole. But 360 degrees on the compass points to Magnetic North in Canada. If you haven't applied magnetic variation, you could be flying as much as 15 degrees or so off course.
Runway names are the headings given in degrees magnetic, but as magnetic north is slowly moving, occasionally the runway name (eg 35/17) will change.

The true north pole, aka the celestial north pole, is the point on the Earth's surface intersecting Earth's rotational axis on the northern hemisphere (and thus the axis around which all stars appear to rotate). Prior to the introduction of the Global Positioning System, there were no perfect indicators of true north.
Two competing and often conflicting definitions of north have been used prior to the advent of GPS (and since). Magnetic north is the direction in line with the Earth's magnetic field, thought to be caused by the convective flows of liquid iron in the Earth's outer core, which causes a compass to point toward the magnetic north pole. An approximation of "celestial north" is in the direction of Polaris, which is a fairly bright star in the night sky and also the closest such star to Earth's rotational axis (and thus "true north") for about the last 1500 years.
First, the two poles do not coincide; here's a Google Earth image showing true north in green and magnetic north in red:

As you can see, they're close, especially from the perspective of those of us in the New World, but if you were sailing off the west coast of the British Isles, you might see up to a ten-degree difference between your compass and Polaris, and that's significant over a distance of even a few hundred miles. Navigators have known of this difference, known as the "magnetic declination", for centuries, and it was a secondary driving reason for the solution to the "problem of longitude" (how to determine, quickly and accurately, your current longitude; knowing this, in addition to being a key coordinate of your position at sea, also allows you to determine the necessary amount of "declination" from magnetic north to determine true north and thus the correct magnetic heading to your intended destination).
Second, Earth's magnetic pole changes over time; here's a map showing the changes in the surveyed position of the magnetic north pole through Nunavut, Canada over the past 130 years or so:

As you can see, the magnetic pole appears to be approaching true north, possibly due to gyroscopic stabilizations of the convective flows within earth's liquid inner layers. Again, for most of the U.S. the practical effects of observed shifts are minimal, but it has much more pronounced effect near the Prime Meridian which happens to be nearly perpendicular to the line between true and magnetic north.

Actually, on the west coast of USA, the effect is much more pronounced than in UK. In London, magnetic variation is a little less than 1°W while in Seattle it is almost 16°E! Because the magnetic field is not very regular. See https://en.wikipedia.org/wiki/File:World_Magnetic_Declination_2015.pdf

True north (geodetic north) is the direction along the earth's surface towards the geographic North Pole.
True geodetic north usually differs from magnetic north (the direction a compass points toward the magnetic north pole), and from grid north (the direction northwards along the grid lines of a map projection).

Magnetic declination is the angle between compass north (the direction the north end of a compass needle points) and true north (the direction along the earth's surface towards the geographic North Pole).
So to get Magnetic Heading, look at your compass. To get the true heading, you will need to look to a WAC chart, and depending on your position, add or remove degrees according to your position's Magnetic declination .

Magnetic heading works with a compass. No need for electricity, no need for a radio, no need for satellites. If you have a failure of any navigation system, or main battery, your compass will tell you the magnetic pole (which is slightly different than the real pole, depending where you are in the globe). It's a safety feature. Satellite work great, when they work, and when your navigation system works. If your battery is dead or something get disconnected, you need some backup that works! You can't say "I lost my battery fuse for my navigation, therefore I got lost and we are going to die" you need a backup, that's why.

True heading is true course ( as plotted on the chart) corrected for the wind.  Period.  Plus or minus magnetic variation gives you a magnetic heading, then compensation for compass deviation gives you your compass heading.  Remember the old adage?...True Virgins Make Dull Company.  Works every time.


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How can I take off in a straight line if the runway isn't visible ahead?
In a C152, flying VFR, if you point your nose up you wouldn't be able to see the runway, thus it would be difficult to keep yourself aligned to it.
So once you take off with a C152, do you climb in a fixed climb rate or do you level off at a certain altitude whilst over the runway, and start your climb again once you've reached the threshold of the runway?

I'm not sure I understand your question fully. First, even if you level off (which IMO is a bad idea for a normal takeoff) then what happens when you reach the end of the runway? You still have to climb in a controlled way, so how are you going to do it? Second, why is climbing in a straight line after takeoff different from climbing in any other situation, e.g. from 4000ft to 5000ft?

In practice, it is easy to glance back through the rear window every now and then to check your alignment.

@Waked That works OK in most Cessnas, but the view out the rear window of my Cherokee is obstructed by the...uh... lack of a rear window. ("You don't want to see what's behind you anyway. That Citation is probably getting awful big awful fast!")

Generally speaking you continue your climb after rotation although this will depend on the runway, runway conditions and airport procedures.
Soft Field Take Off: In some cases if you are leaving from a grass strip (still technically a runway) you will go for your soft field take off maneuver. In this case you will actually pop the plane off the ground into ground effect as soon as you can(you will be flying at this point). Then build up speed in ground effect (at a level attitude so you can see the runway fine) then begin your climb out when you have sufficient speed at a climb attitude.
Short Field Take Off: If the runway is short (or there is an obstacle to clear) you will execute your short field maneuvers. In this case you will take off with 1 notches of flaps. Gun the engine but hold the breaks and let the prop spin up. Then as soon as you hit rotation speed pull back to your best angle of climb. Then when you have cleared your obstacle you can level it out to a normal climb t (best rate or what ever you planed for). In this case clearing the obstacle may be more important than strictly maintaining the runway heading (depending on where the obstacle is). In this maneuver you will focus on your airspeed (to keep from stalling) as well as your attitude indicator and your heading indicator to keep as on course as possible.
Air Port Procedures: Some airports have special procedures for departing because of obstacles, air spaces, noise abatements etc. For example I fly out of DYL and they have a procedure on runway 23 that calls for a turn to 250 and climb out to 1200 ft before executing any other turns or maneuvers. Once I have cleared the trees I will usually begin my turn.
Generally speaking you fly the runway heading after you have rotated and you do this using the heading indicator NOT the compass. Since you are in accelerated climbing flight your compass is inaccurate.

Thats what the compass is for.   Continue to fly runway heading according to the compass & heading indicator.
While you may not be able to see the runway, you may have other markers.   Even a good cloud in front of the airplane can serve as a VFR reference point.
Or look out the side to the left-and-right of the plane.  You should be able to tell if you're going straight or turning by what you see going past the sides of your plane.

So I guess a good cloud is the one which doesn't drift away, and stays still until you reach the desired altitude!

I don't think I've ever used a compass or heading indicator in a VFR departure, and I don't think it'd work all that well in a crosswind.  As you say, you look out the sides of the plane.

@jamesqf The compass works OK in a crosswind: Runway heading is still runway heading. If you want your ground track to stay aligned with the runway it still works, with one caveat: Rather than flying "runway heading" you fly whatever heading you crabbed around to in order to remain over the runway while you could still see it (which gives you a ground track aligned with the runway, give-or-take wind speed variations as you climb).

When flying a tailwheel airplane or a tandem (two seats, front and rear) with a passenger in the front seat, you can't see much of the runway at all either landing or taking off. I use my peripheral vision to keep the plane centered between the edges of the runway and on track.

Yes. I had a couple of rides in a Stearman. The combination of tailwheel, tandem cockpit, radial engine, and biplane wings makes for terrible forward visibility. Watching the runway edges was the only way to stay on the runway. Landing was done with a turning approach almost all the way to the threshold with a slip once aligned.

"terrible"? non-existant!

Indeed, that's probably a better way to put it.


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How does the environment influence the aircraft engine demanded thrust?

When engineers design the power management of the FADEC, how do they set the required thrust when the pilot pushes the throttle to the TO/cruise/idle position?
I have read some papers, and I found that the demanded thrust is related to the inlet environment, but I don't know how to calculate the influence of the environment, and how to correct the thrust.

Welcome to Aviation.SE! "How did they set the required thrust when the pilot pushes the throttle to TO/Cruise/Idle position": This would be a good base for a good question. The environment part is less clear, and maybe should be treated separately in a later question when you have the answer about the settings.

Yes, you are right .I would change the order of the questions.

All modern engine control systems take 'environment' (most importantly inlet total pressure and temperature) into account.
Takeoff and maximum continuous thrust are normally limited by the turbine temperature (which is controlled internally) and N2 (high-pressure rotor) RPM. So such settings on the throttle usually preset a certain N2, and then the thrust will be whatever the engine develops in present conditions. However, this limiting N2 may, and often does, depend on the environmental conditions. The max max (takeoff) is usually a hard limit, but the max cruise N2 may increase with altitude (falling temperature/pressure). Thus when you are climbing with a constant throttle setting, the control system may increase the RPM - even though the actual thrust will be falling due to engine physics.
Idle thrust is limited by stability of the flame. Idle RPM on the ground may be substantially lower than what can be sustained in the air (at speed). Thus the engine control systems often have a weight-on-wheels input signal and apply different control on the ground, primarily with regards to idle power.
Idle can also be significantly affected by the amount of bleed air taken from the engine, but this is not quite the 'environment', I suppose.
Of course, real control systems are a lot more complicated, even if we consider just the environmental parameters. For example, they can have numerous logical rules like: "if idle is selected after having maximum thrust for more than X seconds, ignore the pressure sensor and hold its value for Y seconds", or some such.

what i say is "required thrust",which maybe given by aircraft, not the thrust produced by aircraft engine.

@LMerCy, then I don't quite understand what you are asking. When pilots set, say, TO thrust, they in fact request ~100% RPM (sometimes 101% or some such value) from the engine. What thrust actually develops depends on many environmental factors. Before takeoff, pilots should consider all these factors, and their manuals/computers may advise to use reduced thrust. But in this case they (or the autopilot) will set a reduced setting on the throttles, so it won't be a TO power. Even if autopilot does it, this is generally not the domain of engine control system.

Thanks for your reply. I think when pilots set TO thrust ,the aircraft engine should produce the required thrust to ensure the plane can take-off safely which is defined by aircraft ,I am not sure whether the RPM is 100%(maybe when they design the aircraft engine to ensure when the aircraft engine produce the TO thrust the RPM is 100%).Then that's my opinion is the requirement of aircraft is first which the FADEC must meet .

@LMerCy, generally, no, the engine controller doesn't know much about what the airplane needs. It usually doesn't know its weight, let alone drag or runway length. It is the job of the aircraft designers to match the engine to the airframe so that enough thrust was produced in the defined set of conditions. It may happen (say, in hot weather) that even TO thrust will not produce enough, and takeoff won't be possible. It is then responsibility of the pilots to set throttles (or program autopilot to do so) to the most appropriate setting, or in some cases not to attempt takeoff.

Could u please give me the source about you said " engine don't know what the airplane needs" ?


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In case of failure in the Electronic Flight Instrument System, how would they be transferred between each other?
In EFIS, in case of failure in the PFD, where would be PFD shown? In case of failure in the ND, where it would be shown? In case the PFD & ND both fail, where would they be shown?

The pilot and copilot PFD's are separate systems, if the pilots PFD fails, the copilot PFD should still be operational. The screens can be switched on either side but if they both failed for some reason the pilot with working displays will probably assume control.

Are you asking about failure of the display itself?

Yes, I am asking about in case of one or two of these displays fail what is the situation ?!

I've seen document on captainpilot.com, which described this for A320 too, but the site is down.

On the Embraer 170/190-series, the normal display layout is as follows:

There is an automatic display reversion logic when one or more panels fail. The following image shows this logic:

There is also a manual reversion switch located near each pilot's outboard knee, which controls what is displayed on the panel:

All images sourced from an Embraer Airplane Operations Manual.

What will the MFD contains in case of failure ? and can you label the priority of the displays from the Most significant until the Less significant displays ?

A failed display will be blank. As you can see by the system logic that the PFD is given the highest priority, then the EICAS, and finally the MFD.


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How does one train for making landings and takeoffs in bad weather?
I thought of this while coming in on a bumpy flight into Memphis, TN. The pilot of the airliner brought in the aircraft nicely, despite having to fight it quite a bit during the slowest portion of the approach. Now, Memphis is fairly flat, and the airport isn't near any large buildings so this isn't too much of a problem, but how do pilots practice bad weather approaches and takeoffs? It's kind of dicey to drop a newbie pilot into swirling bad weather, and I know as the aircraft gets smaller handling rough weather becomes harder. Is it done purely through simulators or do they use training flights where they intentionally fly during really rough weather? How does this training differ between general aviation and airline pilots?
I'd imagine flight simulators are good up to a point, but may not simulate all of the bad emergencies that could happen in bad weather (choppy crosswinds, hail, rain, lightning, etc.) so to train properly one would have to go up on training flights. However, I know that is very dangerous, and probably expensive, so it wouldn't be preferable. What platforms do current pilots use to get proficient at bad weather flying?

Modern simulators are very extensive and do simulate all kinds of bad weather conditions. I had the opportunity to fly a 777 simulator once and skidded way off the runway when the operator made it snow (not just gentle snow, full on blizzard) with a strong crosswind.

Sadly, simulating real weather related accidents is a great training aid. That final culmination of the real thing is an excellent reference point for understanding the chain of decisions and actions caused a world of hurt, or could have made for a different outcome.

Your question has two components: 1) how do pilots train for adverse conditions and 2) how do pilots stay fresh on adverse conditions?
As you said, smaller airplanes (really, slower airplanes) are hardest, because the wind speed is a larger percentage of the airplane's flying speed. There are no real simulators for small airplanes: we learn by going out and doing. When you first learn to fly, your instructor makes sure you can handle crosswinds and gusts. With the Instrument Rating comes more adverse weather.
Once you have learned the techniques (and stay proficient by practicing regularly), faster airplanes and higher winds are not really a problem -- the same techniques apply, and in any case you will have a more experienced pilot or instructor helping you along.
I must call out your use of "newbie pilot" -- I hope you're not implying that airline pilots are newbies in any sense of the word. Before you can even hope to get an airline job you've already experienced bad weather in marginal airplanes.
Now, staying proficient: this is where transport airplanes (airlines) are very different from the small airplanes. The airlines' pilots practice regularly on simulators that really can duplicate just about any kind of weather you'd fly in. Pilots of small airplanes though... the simulators can't do that, and we practice in the real world.
All that said, a passenger's idea of what makes a good landing is very different from a pilot's! A passenger thinks a smooth touchdown is what makes a good landing. For a pilot, a good landing means a stable and controlled approach followed by an appropriate touchdown that maximizes safety -- in gusty crosswinds a firm touchdown is much safer than a greaser.

A firm touchdown is strongly preferred on a contaminated runway to mitigate against hydroplaning, and required on a max-performance landing to maximize runway available for braking.

1 for correcting the notion that ATP's might be newbies.

Besides simulator, pilots usually pair up with one another in airline operations. There is the Captain and the First Officer, the Captain (most of the time) being more experienced. The First Officer can observe the decision making process of the Captain, e.g. why do we use a certain flaps setting, why is it ok to pass through that storm or should we abort the landing, etc.
Storms, or snows, while not uncommon, are not something you see everyday. That's how experienced is passed onto new pilots: by flying with a more experienced pilot who has seen these weather many times.
As far as skills is concerned, there aren't many new flying skills in an airliner compared to a small aircraft, except for maybe a few tips or tricks dependent on that specific model. Simulators can provide some quite extreme weather for training; but unless you've been super lucky, you are likely to have encountered unfavorable weather on your way to becoming an airline pilot.

Are emergency situations practiced using the buddy system? (like stalls, sudden downdrafts, etc)

Sounds like fodder for a whole new question, @user11377.


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How can an airliner have low altitude stall recovery?
I was going through the air crash record and found out that a lot of them happened when the aircraft stalled at a low enough altitude where the pilots couldn't get enough speed to pull back out. I was curious to see why this was the case when I learned about flow separation hysteresis where to reattach the flow the wing needs to pitch down further than the static stall angle. Reading that and the crash record brought up a question in my mind:
How could an aircraft be engineered to have safe low altitude stall recovery? (aside from having a VERY low wing loading, ballistic parachutes, or a computer to prevent pilots from entering low altitude stalls)
From a cursory analysis, this situation seems ideal for some form of emergency engine, either electrically powered fans or a few solid rocket motors to right the aircraft and provide enough power to at least make the touchdown not a crash. Or the wing and stabilizers have some form boundary layer acceleration to reduce the amount of time needed to recover from a stall.
I'm curious to see what approaches have been tried to try and solve this problem or if the newer aircraft have some other technology incorporated.

The most important reason for aircraft stalls is due to high angle of of attack, which causes flow separation.
Usual stall recovery procedures consist of reducing the aircraft angle of attack (pitch angle) and increasing power so that the stalled wing starts producing lift and the aircraft forward speed is increased to the required amount. Then the controls are applied in such a way to return to level flight as soon as possible.
The problem with low altitude stall is that stall recovery usually involves some loss of altitude and if the altitude is low enough, recovery from stall may not be possible before contact with ground.
However,the recovery altitude may be decreased to a certain extent by incorporating the following technologies in civil aircraft.

Use of high lift devices- Use of high lift devices like slats and flaps increases the lift coefficient and also delays the onset of stall. In case the slats have not been deployed before stall, they can be deployed to get the airfoil (wing) produce lift again.

Source: zenithair.com

Increasing the power- In general, greater the power applied, the less the loss of altitude. In this case, the engines usually have excess emergency power that can be applied for reducing the loss of altitude.

In general, it is better not to enter into a stall and most of the technologies are geared up towards prevention of stall than recovery due to a number of reasons

In case of civil aircraft, below a certain altitude, stall recovery is usually not possible.
Addition of systems to ensure a soft landing in case of stall increases the cost and complexity and in any case may be impracticable in case of large aircraft. As such, it is better to train the pilots to prevent entering into a stall in first place.

"In case of civil aircraft, below a certain altitude, stall recovery is usually not possible." That's much more the case for jets than for piston-powered aircraft, though, due to the much longer delay before power is available in jets. Piston-powered aircraft can get up to full thrust rather quickly, making stall recovery possible more quickly with less loss of altitude.

The main solution to speed up stall recovery has been to install more powerful engines.
Stall is a total energy problem. If the aircraft is low, it has little potential energy left to top up its kinetic energy, and a stall is exactly when the kinetic energy drops below the value needed for staying in the air.
As you correctly note, the boundary layer needs some time to recover, and the further back on the wing you look, the more the boundary layer is shaped by what went on before. This helps to delay stall when the pitch rate is sufficiently high, but makes recovery take longer. Boundary layer suction would certainly help, but all cases of suction which I know of were either installed to keep the flow laminar for longer or to delay separation at high angle of attack. The suction volume needed to speed up stall recovery is a magnitude above that needed for delaying stall, so even when suction was installed, it would have been insufficient for shortening the stall recovery.
The idea of quickly adding kinetic energy is much more promising, and your proposal of a rocket engine will certainly help. If it points slightly upwards, it can compensate for the lift loss during stall and speed up the airplane so it returns quickly to the linear flight regime. The same can be achieved with sufficiently powerful engines, and since they are much more fuel efficient than rockets, they have been preferred over rockets to make fighter and aerobatic aircraft essentially unstallable if their thrust-to-weight ratio approaches one.
Next, a benign lift curve slope at stall can be engineered into the wing by using a big nose radius, a linear pressure recovery gradient and washout, so the outer wing still has attached flow when the inner wing stalls. The first two factors help to keep lift fairly constant well into the stall, so vertical acceleration is low. The third factor ensures proper aileron response and low roll moments due to flow separation, so the aircraft will not divert and stays controllable. The drag increase due to separation, however, will decelerate the aircraft and lead to a loss of lift if the stall is not ended quickly. The benign lift curve slope of the wing made the stall in case of AF447 so uneventful that the copilot never realized that he had pulled the aircraft into a stall.
However, most aircraft are flown such that they never enter into a stall. Instrumentation and warning devices will prevent even incompetent pilots from stalling the aircraft unless they intend to do so, eliminating the need for emergency rockets or oversized engines.

I know, the last sentence will provoke comments which mention AF447, but I distinguish between incompetent and grossly incompetent pilots. As you know, it is impossible to make anything foolproof because fools are so ingenious.

What response times are needed to recover from a stall at low altitude? I ask because gas turbines usually have some lag to adjust throttle settings due to the rotational inertia of the rotors, on the order of 1 second if I remember correctly. Is that fast enough?

@user11377: You are right, you need to go into the stall with some thrust already. Spooling up from idle will take several seconds and will not help much to speed up recovery. Also, much depends on how deeply the airplane is flown into the stall. If the pilot gently pulls and releases stick pressure as soon as he gets the first stall indication, recovery is instant. If the pilot pulls up quickly and then waits before correcting, recovery will take much longer.

As soon as you invent something foolproof, nature produces a better fool.

The nice thing about rockets vs. more powerful jet engines, though, is that rockets start producing maximum thrust almost instantly, whereas jet engines take several seconds to get there, as you noted. Of course, the jet engines will be far more efficient (and significantly less likely to explode.)

@reirab: Ah yes a little theorem. You cannot stall the plane while possessing a TWR > 1.

@Joshua Sure you can. All VTOL airplanes do it. :)


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Why are flight simulators bad at predicting drag and lift values at high angles of attack? (the nonlinear flow regime)
X-Plane is built on something called blade element theory. From my understanding, it says the aircraft performance can be found if the performance of 2D cross sections are calculated and integrated in real time. Laminar Research (the maker of X-Plane) uses that as a selling point for X-Plane, saying it can be used to help aircraft designers try out their designs in flight on the computer before building prototypes. However, as commentators on this forum have written before (Peter Kampf), most flight simulators are bad at predicting the performance of the airframe at high angles of attack without using a table of force coefficients generated through windtunnel data or detailed CFD.
My question is, what in the flow physics drives this? What part of the physics is modeled incorrectly to give garbage data at high angles of attack?

The difference is in matching the outcome of the physics model with real measured flight data, which is indeed proprietary and very expensive. You can model anything in many ways, but you need to verify what you've modelled is according to measured real world data.

@Koyovis This is not what the question is asking. For laminar flow, you'll find that it is very easy to match computations with real world data; the question is asking why this is not the case for turbulent flow.

@ Sanchises. Yes indeed. And real flight has vast regions of aircraft skin with turbulent boundary layer. In an airliner in cruise flight, transition from laminar to turbulent for the fuselage is right behind the cockpit, as sound measurements show.

Laminar flow is easy. While there is a complete set of differential equations that describes any fluid flow, there is a wealth of simplifications and assumptions that you can use on laminar flow. This means that X-Plane doesn't need to model all the air around the wing, but can do the calculations based on wing profile and local velocities. Everything is relatively linear, so you can just change some variables and use predetermined parameters. Furthermore, it's completely time-invariant in steady state. You can now easily solve the equations, integrate the calculated pressure profile along the wing, and done!
For turbulent flow, none of that holds. The current way of simulating turbulent flow is to do a Finite Element analysis or similar (e.g., FDM). Basically, you will need to consider all the air in a large volume around the wing, divide it up in a small grid and simulate it. For a good calculation, this takes in the order of seconds to minutes for a single 2D cross-section on my pretty decent laptop. And then we're neglecting the 3D influences. Furthermore, turbulent flow changes w.r.t. time. For example, open your car window on the highway - you will hear the wind roar and pulsate. This means that you will need to get your previous pressure- and velocity field, and use that as a starting point for your next FEM/FDM analysis. Finally, turbulent flow is extremely hard to predict correctly, even with above methods: a slightly rougher surface, a small bolt or a small wind gust can delay flow separation for a few inches, completely invalidating your results. Perhaps this YouTube video (note: this is not simulated in real time!) might shed some light on the vast complexity and time-dependency of turbulent flow - and remember that your horizontal stabilizer will see disturbed air in stall conditions, making it necessary to simulate the entire flow field around the airplane for a correct simulation, not just the wing sections.
Of course, X-plane has a framerate measured in frames per second, not frames per hour. This means that they use a bunch of assumptions to calculate the wing lift. I'm guessing they just have some values for location of flow separation and turbulent pressure for some fixed velocities and angles, and interpolate to the actual values. My guess is that they also have some parameters that aren't actually calculated, but are chosen such that some very basic maneuvers are possible like spin recovery, regardless whether these values correspond to any actual physical phenomena - it's a game, after all.

What about purpose-built simulators? Do they simulate stall conditions any better? They aren't run by supercomputers after all.

My best guess would be that purpose built simulators just have a lot more extensive lookup tables based on windtunnel tests, but these are quite expensive and probably not public domain.

Within a certain envelope, the professional simulators match a set of measured data quite well. In fully developed stalls this is no longer the case.

@Koyovis Indeed, that would require putting aircraft in fully developed stalls in a wealth of attitudes, which is not really something you want to do (especially larger aircraft)

Because real airflow is not 2D laminar.
Not sure how far along we are with 3D turbulent flow CFD at lower angles of attack. But with high angles of attack the airflow separates from the stream body and creates a wild and random pattern of flow fluctuations - not the best candidate for CFD.
Level D full flight simulators must match a set of data that is measured on an actual aircraft. This set of data is around reasonable flight conditions, such as usually encountered during airliner revenue flights, and including most emergency situations that pilots are trained in.
Pitch AoA is measured between about -2º to stall onset, about 25º. and +/- 15º of sideslip, but not at combinations of extreme AoA and sideslip. The region that is measured during flight tests looks like the green are in picture underneath, which are the flight states in which Level D simulators must very closely match the flight data:

The blue and yellow boxes are interpolations and extrapolations of the table lookup data used for the Acceptance area. This is sufficient to train entry into stall, but not sufficient to train recovery from a fully developed stall such as what happened with AF 447. The FAA will introduce a requirement that pilots train recovery from a fully developed stall, and Airbus and Boeing are working on updating their data packages.
Some data of crashed aircraft flight recorders and wind tunnel data have been used to model stall behaviour of different aircraft types (low wing, T-tail etc), and the first simulators are already operational that can train this. Alaska Airlines has one.

Please add a source for your images. and personally I find it a bit unintuitive to read, it almost looks like it is saying that the loss of control is inside the red perimeter, while it obviously is outside (and why is it asymmetrical?)

Yes - it's sales information from a company that specialises in flight characteristics out of the normal envelope. The green bit is very extensively measured & tested. A stall approach exercise takes place in the red envelope, starts at 45 deg AoA 20 sideslip and successfully ends at 0,0. Meanwhile we've crossed interpolated and extrapolated data, so how valid is our training? An illustration of why we need full 360 degree data if we want to train fully developed stall recovery - but the industry only moved there after Air France 477 and Air Asia QZ8501.

There's a lot I don't understand about that diagram. What does the red dotted curve indicate? Is it the boundary of some region or does it show a path which is traced during some process? What does "acceptance" mean in this context?

@ Tanner Swett yes indeed, I'll update the answer. The dotted red line is indeed an exercise trace. Acceptance is the region where flight simulators must demonstrate how tthey match actual flight measurements.. By necessity the measurements are taken within boundaries of normal flight.

I'm sorry, I can't answer the physics part. It may be a good question on Physics StackExchange.
On a programming point of view (SuperUser ?):

Floating points calculations are among the most CPU intensive.
Precision matters when you talk about physics behaviour. Without enough precision, some parameters may jump from one value to an exaggerated one, even to the opposite value (stackoverflow), even if it's the next available value that can be stored in bits. All a computer can do are approximations.
Cycle update/refresh cycle, or interval duration between states changes in physics parameters. 10 milliseconds may look really small and good at simulating physics properties. However, bring the aircraft in stall conditions and you realize that 10ms is not enough to accurately resolve all surface components efficiency on a normal computer. That's why on some simulator, especially the old ones, you have unexpected aircraft behavior like brutal rolls/pitches/yaws.
Race condition. This is a situation of concern on parallel tasks made by the computer. Simply put, one task is to calculate vector forces applied on the aircraft. Another parallel task is user or autopilot inputs. And another task is to generate new airflow conditions, like gusts and turbulences. Those are not synchronized. The more the tasks, the othen asyncronuous computations occur.
A computer just does what it has been taught to do. The aircraft can break if the value of one parameter goes beyoud a certain limit, and a brutal overreaction is induced by one miscalculated parameter (due to the refresh cycle above, or obsolete parameters due to the async multi tasks) Some damping fallbacks checks can be added in the code (like on computer driven pilots inputs), but this also add more computations to perform on user computer where the simulator is installed.
Random update. To make turbulences for instance, you have to create dynamic random parameters whose bounds changes depending on weather conditions, altitude, geographic locations, etc. This adds to the overall computations to perform.
By the way, the simulator has to render graphic datas on screen and handle thousands objects with millions of parameters. That's also why, unlike X-Plane of FSX/P3D, real professional simulators doesn't care much about sceneries objects (traffic) and other eye candy like clouds beautifulness or sun glare. Because physics computations are prioritized.

It's not that the datas are corrupted or miscalculated. They are 1) approximations as results of very simplified formulas or lookup tables/matrices to speed up computation time, and 2) are not always updated at the very moment they are most required (high AOA) because of excessive task to perform to update them. Adding a parameter (like an extra wing fence above the wing) and computations to perform on each update may increase linearly or exponetially. The physics model is not incorrect, but is not the most accurate one either.
In the end, one can get near to the best possible approximation, but at the cost of computation time. We often hear a computer took x weeks to calculate y trillions decimals of Pi, or z days to display an accurate overview of our galaxy colliding with M31 Andromeda. Getting there is possible, but not yet through mass public simulators like X-Plane, FSX or P3D. Common computers are not powerfull, fast and precise enough yet, to reproduce realtime physics behaviors that complex like aerodynamics or gravity.

Race conditions do not make sense in this context. They refer to a bug when it's assumed that a value is not yet modified when it actually is, not having a 1/60 second delay in your autopilot input because the new position vector is not yet calculated.As for turbulent flow, the physics model may actually be incorrect (and not just 'not the most accurate'), because a decent approximation would be too resource-intensive, or simply not yet researched for a specific wing profile.

@sanchises: Noted, thanks. Though, on a n-body object problem we had some years ago, no matter how we splitted the threads, it was impossible to get the required datas in time (matters of split seconds) We even tried performing the task on a second computer, but hardware limitations worsen it. Yes it's a bug in the way the target result was impossible to reach in time. Solution was to sacrifice parameters to kill the delay, and it's in that way I underlined the fact models are indeed wrong in the first place, I agree, but acceptable in a certain extend (lambda computer vs dedicated ones)

Ah, like so. I'm still not sure that's called a race condition though. But either way, it will indeed slow your simulation unacceptably. We'll just wait for Moore's law to catch up :)

Cloud and weather simulation are in integral part of Level D simulators, projected on collimated mirrors with an uninterrupted view of 200 deg H x 40 deg V. Update rate 60 Hz. Have you ever been in one?

Floating point performance is not an issue at all, the complete physics model of all systems, aerodynamics, flight dynamics, ground reactions, engines etc etc can easily be computed at 2000 Hz by a simple modern laptop. The graphics is the bottleneck.

I cannot answer for which CFD model (or strategy/mix thereof) is used by any of the flight simulators.
I can say that CFD, in general, is more limited by available resources (FLOPS, and time) than it is about precision. At least, this is the case for a variety of situations in which most flight simulation occurs.
Realtime flight simulation (today) must make use of shorthand 'parameterizations' of the simulation grid, in order to resolve model forces at the FPS of the user experience. If the COU (and potentially the GPU) could perform more ops, then the simulation grid could use boxes that are smaller – thus higher resolution and more accurate.
Higher spatial resolution simulation grids, along with higher temporal resolution grids, are able to better capture small flow defects. These lead to changes in the flow topology, compression waves, boundary layer separations, and other more complex nonlinearities.

There is an extensive set of actual flight data required for a professional flight simulator, which must be precisely matched in order to certify the device. FLOPS is not really a problem anymore: a full set of physics models for a Level D sim can run @ 3000 Hz on a simple laptop, while only 100 Hz is required.

@Koyovis The question is asking about a simulator using CFD rather than model-based physics.


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Why can't model aircraft be used to discover spin and stall behavior of manned aircraft designs?
I was reading some literature on stalls and spins (which are almost always preceded by a stall) of aircraft when a thought hit me. Why can't a model aircraft (1/4 scale of manned aircraft) be used to inexpensively explore the stall and spin behavior of manned aircraft?
(One disclaimer for this, I know this can't be done at ALL speed ranges or all scales. Airliners are too big to make inexpensive 1/4 scale R/C aircraft, and aircraft that cruise at transsonic speeds will not benefit from this since the effects seen there only happen at that particular reynolds and mach number combination)
However, I was thinking more for aircraft that spend a lot of time at fairly low speed (below Mach 0.3) such as ultralight or light sport aircraft. Here the air is effectively incompressible, meaning flow physics wise, the reynolds number is dominant in determining flow separation. And because of that, all a 1/4 scale model has to do is fly four times faster to get the same flow separation characteristics as the full scale aircraft.
So for an ultralight aircraft, which is speed capped at 30 m/s, a 1/4 scale model would have to fly at 120 m/s to see similar stall behavior, which is at the bottom end of where the flow starts to become compressible, meaning most of the flow structures seen at full scale will be seen at the model scale. Accounting for the lower moment of inertia of the model, the spin behavior of the model should predictably scale to reveal the behavior of the full scale aircraft.
My question is, what am I getting wrong in my reasoning? Why can't this work?

Spins are "almost" always preceded by a stall?

I spun my model aircraft right into the ground the other day :-/

Simplest answer; air doesn't scale. An exact scale replica of a larger manned aircraft, with accurate length, width, wing dimensions, mass, CG etc, will still have different airflow characteristics that will skew the results. Despite this, models are often used in wind tunnel testing to determine basic aerodynamic properties; the subtleties, however, must be learned closer to full-scale.

@SteveV.: A few aircraft have sufficiently high rudder authority that you can enter a flat spin simply by stomping on the rudder and holding it; most post-World War I aircraft have to be stalled to spin, however.

Who says scale models cannot be used for spin modeling?
The key information is that the Reynolds number does not affect the behavior of separated airflow as much, so you do not need to increase speed to compensate for the smaller dimensions. Only when the stall behavior is studied would the Reynolds number matter, but here also meaningful results are possible at lower speeds.
Models are routinely used in spin tunnels. Here both the speed and the dimensions are smaller, but the results can be transferred to the original aircraft. There are two sorts of spin tunnels:

Free tunnels with an open, divergent section in which the model has to be thrown by a skilled operator so it will settle in a spin within the upwards flowing air stream. The section is divergent, so the speed changes with height and allows the model to find a matching speed.

Picture from spin tests on a Grumman E-2 model in the NACA 20 foot spin tunnel (picture source)

Closed tunnels in which the model is mounted on a sting. The sting is connected to a rotating balance, which can be set at different roll and yaw rates. The resulting matrix of coefficients over angle of attack, angle of sideslip and over the three rotation speeds is fed into a computer model which computes the equilibrium points.

The models used in free tunnels must be scaled both geometrically and inertially, so their mass distribution matches that of the original. If the spin tunnel uses a rotating balance, not even the dynamic scaling of masses and inertias is needed, and a regular tunnel model can be used. However, if the resulting spin is of an oscillating nature, the free spin tunnel is at an advantage, because this becomes readily apparent in the test. In a closed tunnel you just get two equilibrium points and must make the connection yourself.
A third way are free-flight tests of models, but they are much more expensive and allow less observations than wind tunnels.
The stall characteristic is a little harder to predict using models, but again you can draw conclusions from what can be seen in the tunnel. To arrive at the maximum lift coefficient starting from the measured value at Re = 1,000,000, you may use this scaling: $$\Delta c_{L_{max}} = \frac{log_{10}\left(Re^{\frac{1}{6}}\right)}{3.5}$$
The plot below shows the difference in lift coefficient between a small model and a full-scale airplane. The biggest differences are around stall, and at the high angles of attack seen in spins both show fairly similar behavior.

Lift coefficient over angle of attack for model and full-scale aircraft, taken from Joseph Chambers' monograph on testing with models (Modeling flight : the role of dynamically scaled free-flight models in support of
NASA’s aerospace programs).
For small aircraft, the relative expense of a tunnel test is normally inadequate. Instead, the real thing is used. Since the speeds involved are small, spin tests are carried out with a spin chute or a releasable mass at the tail of the aircraft. On modern designs, even a full-aircraft ballistic parachute system can be employed to prevent a crash if the spin test ends in an unrecoverable situation.

Spin chute installation on a Columbia 400 (picture source)

Scaled models are being used to determine spin characteristics of manned aircraft. For example, see this image from Modeling Flight- The Role of Dynamically Scaled Free-Flight Models in Support of NASA’s Aerospace Programs by Joseph R. Chambers:

However, there are significant difficulties in correctly modeling the spin characteristics of the aircraft.
For a model to properly simulate a full scale aircraft, both should be dynamically similar- i.e. the flight path and angular displacements of the
model and the aircraft will be identical (geometrically)though the timescale will usually vary.
For the model faithfully reproducing the aircraft spin characteristics, the (balance of) aerodynamic and inertial parameters should be similar. The model should be scaled (in addition to the geometrical parameters) in the mass, moment of inertia, linear and angular velocity etc. For example, the scaling factors in case of incompressible flow are:

Image from Modeling Flight- The Role of Dynamically Scaled Free-Flight Models in Support of NASA’s Aerospace Programs by Joseph R. Chambers
In case of compressible flow, still more scaling is required:

Image from Modeling Flight- The Role of Dynamically Scaled Free-Flight Models in Support of NASA’s Aerospace Programs by Joseph R. Chambers
The issue is faithfully duplicating all this scaling is not possible; you are correct that the Reynolds number is the most significant issue involved. Spin involves (partially or fully) separated flows, which are significantly affected by the Reynolds number.
The above mentioned report concludes:

The data show significant increases in the magnitude and angle of attack for maximum lift as Reynolds number increases from model conditions to the full-
  scale value. Such results can significantly affect the prediction of flight characteristics of the airplane near and above stall.

Another issue is the cost of creating a model; There are significant costs involved in correctly modeling and testing (at-least drop-testing) these systems, the resources for which are not available for all but the most expensive programs. For example, The Spinning of Aircraft- A Discussion of Spin Prediction Techniques, Report no AD- A216- 200 concludes:

The drop-model technique using approximately 1/4 scale-models ... has the potential to cover all phases of the spin. However, because of cost and substantial manpower requirements it only
  becomes viable for major projects.

These are the reasons we don't see scale models being used to model the spin characteristics of GA and ultralight aircraft.

"No, sir, that F-15 isn't ripe yet, you'll have to come back in a year or two to pick it."


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What technical problems stopped scramjets from being used on hypersonic aircraft?
I have been researching hypersonic aircraft and scramjet engines after running a few calculations looking at ramjet/rocket hybrids for making single stage to orbit vehicles. After doing the analysis and reading a little bit of the development history, I am still a little baffled as to what stopped the development of the engines to get deployed on aircraft and rocket.
What is keeping the engine cool enough to not melt or fly apart at design speeds? Was it making sure the inlet compressed the air quickly enough without slowing the flow with a normal shock? Was it making sure the structure of the engine was light enough to still produce enough thrust to fly the aircraft?
If these were the problems, why were they hard to solve? Was it that we did not know how to get enough cooling for low pressure drop? Or know how to locate shockwaves in the intake because the ratio of specific heats changes?

Has a scramjet run for more than a few seconds?

Why do you think development has stopped?

I meant historically. I know the effort hasn’t stopped, but was asking why we don’t have an engine ready to put onto an aircraft right now.

I'm don't research hypersonic flight, but I bet the primary barriers of hypersonic flight are less technological and more political/economical. After all, we pay money and conduct research in order to solve technological problems.
You would think that developing an engine is primarily a task for aerodynamicists since they can design better intakes, compressors, nozzles, etc. However, materials science will typically limit what an engine manufacturer can actually build as you mentioned in your question. Let's imagine aerodynamicists design can design a scramjet that can reliably run for a thousand cycles, but the engine is useless if the aircraft's skin can't handle the high speeds.
More importantly, who's willing to pay for super-inefficient scramjets? Commercial airliners (consumers) weren't even willing to pay for the supersonic Concorde, so you can easily discount them from buying scramjet-powered aircraft. That leaves the military. In the 60s, the military was willing to pay for speed (see SR-71, XB-70), but then stealth became more attractive than speed and super/hyper-sonic research fell by the wayside.
Now the US, China, and Russia are again funding hypersonics research in part because stealth is losing its advantage. Moreover, some startups (Aerion?) are exploring supersonic business jets for the really rich folks, so you're starting to see their engineers exploring hitherto exotic supersonic tech. However, hypersonic flight will still be beyond the means of an average consumer.
TL;DR: Hypersonic research probably stopped less because of tech problems and more because stealth offered more bang for the military's buck. Now the pendulum is swinging the other way. Military supersonic technology is also making its way into the commercial sector for the uber rich.

I'd suggest that fuel consumption is another reason why scramjets wouldn't have much practical use in transportation.  As far as I've found, the longest scramjet test flight to date is about 200 seconds.  That might be sufficient for a space launch, or a high-speed drone weapon, but for a practical transport the fuel load would seem prohibitive.


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Can a large aspect ratio wing be actively controlled using distributed electric propulsion to avoid getting ripped apart in wind shear?
This question is related to the recent possibility of solar aircraft becoming flyable. Solar Impulse, the NASA Helios, and the Titan Aerospace/Ascenta Aerospace solar drones have all have issues with making robust structures that can maintain station through atmospheric wind shear. So far, the best approach has been to wait out bad weather before attempting a flight.
My question is this, could the structure be actively controlled using distributed electric propulsion to smooth out the unsteady loads? Where if a section of the wing gets hit with high speed air, the rest of the motors on the wing spin faster to accelerate the flow over the other parts of the wing?
The reasons I can think of this not being doable now are the slow response time for spooling up the motors. And this means getting away with accelerating the bare minimum of air over the wing (basically only the boundary layer). Has this been done yet and shown in flight?

That seems possible, but I think you're right--the response time would be too slow to be effective in rapidly changing wind shear conditions. On top of that, the moments created by the unbalanced thrust in that case may be more detrimental than the wind shear.

Use camber flaps instead. A good example are the A380 ailerons which actively lower gust loads (valse d'ailerons)


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Can a zero hour pilot actually launch a Bombardier Q400 with only simulator experience?
The thief of the Bombardier Q400 at SEATAC stated that he had a lot of simulator flying (I'm guessing like Flight Simulator for the PC?) Some "experts" stated that he must have had more than that to get the Bombardier off of the ground and flying 'stunts' like loops and rolls. Can a zero hour pilot pull off what apparently did happen or did the thief have to have had much more knowledge? I assume that just starting up the plane to taxi would be a complex procedure, is this easily learned in a PC Game?

Related https://aviation.stackexchange.com/questions/54191

Also Can Microsoft Flight Simulator help me learn to fly (or make me a better pilot)? and How close is X-Plane experience to real flying?

Edited the title to clarify how this is not a possible duplicate of how close X-Plane or Microsoft Flight simulator is to real flying or help me fly better.

There is a big difference between "Could someone safely do this" and "Could someone do it once if they got really lucky?"

The hardest thing about flying any airplane is figuring out how to start it. After that, it's pretty straight-forward.

You don't even need your own simulator. Here is a good video of how to start a Q400 (taken from a sim game) and here is one from a tech going through an actual cockpit. There is even a chance there were checklists in the aircraft when he arrived.
To get off the ground he really just needed to know how to gun the throttles and roughly when to pull back on the yoke to get airborne. These numbers can be found pretty readily around the web. Considering his fuel and cargo load and depending on the trim of the aircraft it may have even started flying with almost no control inputs.
Whats important to understand about some of the maneuvers he did was the fact that the aircraft was empty and flying on a partial fuel load which will make it easier to do some of those things. I will refrain from diving deep here until the NTSB report is out but at full throttle planes are capable of quite a bit. Rolling an airliner is not out of the question in the right scenario.

Is the NTSB going to do a report on this incident? There are no NTSB reports on the 9/11 attacks since those weren't accidents, and this wasn't an accident either.

@TannerSwett it looks like the NTSB did publish 9/11 reports that were not released until later I would imagine someone is going to take a look at the telemetry at some point. While the case is an open and shut case from a "why" standpoint there is often always something to be learned from data like this.

Can a zero hour pilot pull off what apparently did happen ... ?

Yes, absolutely.
The hard part about flying isn't the actual mid-flight control, so much as it's getting into the air, getting back on the ground, and dealing with emergencies.
You can learn the first part and the basic flight controls in a simulator. In Russel's case, the latter two parts were greatly simplified: he created an emergency, and he didn't need to get back in one piece.
It's actually possible to learn far more than he did in a sim. The complex part of modern aircraft is the systems, and MSFS X with custom planes is a powerful learning tool. If you haven't "played" it, it's not like a normal game at all. Most of MSFS is looking at the instruments and flipping switches that model the systems as closely as the creators could.
Air maneuvering requires flight time to perform safely, with minimum risk to the aircraft - one has to learn to trust their instruments, deal with their senses, and respect the envelope. Doing it with only sim training is simply dangerous, not impossible.

It is possible to learn and study everything in a (home) simulator except for hand-flying.
Operating an aircraft is (simplified) only about pressing the right switches at the right time and monitoring the aircraft data.
Richard Russel probably didn't do the latter, because the probability that something does not work is low firstly and secondly it wasn't his goal to take care of the plane.
So, it's indeed easily possible to learn how to make this aircraft ready for takeoff, especially if your goal is just to get the engines running. The hand-flying part is something else. But yeah, nobody says what he did was good flying. It sufficed for what he has done.
Also note: He has just done the very basics to get the plane up and fly it. In his conversation with ATC, he asks about how to pressurize the plane and how to operate the autopilot, so he lacks some "deeper" understanding and knowledge of the aircraft. In the background of his transmissions, there also is the master warning audible sometimes.
To summarize: Yes, a home simulator completely suffices for what Richard Russel wanted to accomplish.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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Impact of hurricanes on flights that have to traverse affected areas
Hurricane Florence is forecast to reach the US east coast soon as a very strong hurricane. This makes me wonder if hurricanes have a big impact on commercial flights that have to traverse the area affected by an hurricane?

Related: Do airlines continue operating in the vicinity of a hurricane?

Hm, actually it might not be a duplicate. That question is about airport operations, but this one is about traversing. That effectively means whether the aircraft can cross above the hurricane safely.

I've edited to make this a general question about how hurricanes affect planes that want to cross them, rather than a request for predictions about what Florence will do. I hope that makes the question answerable in a concrete way.

@DavidRicherby We also have a question already about flying over hurricanes

@Pondlife Thanks: I think that covers it.

G Bach, I hope you don't think we've messed you around, here. We can't really predict what this specific hurricane will do, but I think the two duplicate questions do cover most of the aspects you were looking for, at least in general terms. If there's something more specific that you want answered and that isn't covered by those two questions, please do post a new question about that and we'll do our best to answer.


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Does the movement of the magnetic poles affect aviation?
Recently I read that Earth's magnetic pole moves quite significantly, several kilometers. Do these changes affect aviation, i.e. for pilots landing their airplanes?

The moving poles are not the main problem, compasses don't point to the magnetic north pole anyway, they align on the local magnetic declination. While declination also changes with polar axis wandering (39 km per year), this change is minor compared to local magnetic anomalies. For importance of local declination, see Where do I land if I fly from KLAX with a constant east heading, crossing the US and the Atlantic ocean?

Not really. Yes, there are changes, but they are small, regarding the size of earth. So sometimes they have to renumber runway designations, as they roughly represent the magnetic runway heading. And you get updated maps and airfield charts all the time, where the variation is adjusted - among all the other stuff, that changes like frequencies, obstacles, noise abatement procedures etc.
If you live close to the poles, it might affect you harder, but luckily this regions are not that densely populated. I'm from central Europe, here the variation changes about a half degree per year. You don't actually recognize it, considering all the whisky-compasses used in planes are barely usable to hold a 5 degree heading...
Landings are mostly unaffected, as you either use ground based systems like ILS, satellite navigation or make a visual approach. You should croscheck the runway designator with your actual heading to avoid landing in the wrong direction or on wrong runway. But this is not affected by the change of the poles positions. Ignoring a sometimes proposed complete pole flipover, at least...

How about airport in Alaska or in Greenland or in Siberia?

The complete reversal is expected to take thousands of years, so plenty of time to adjust. The increased cosmic radiation due to the weakening of the magnetic field during the reversal is likely to be bigger issue.

@AirCraftLover, those places are still far enough away that the daily and yearly changes don't make much difference. Only really near the north magnetic pole one has to rely on other means of orientation, which GPS makes trivial these days—and runways are labeled with true headings there.

While the position of the pole does wander by a bit, this doesn't cause major problems for aviation for several reasons.
The motion of the pole by a few kilometers sounds dramatic. But given the size of the earth represents fairly small changes in the orientation of the magnetic field.  Away from the poles, these changes are much harder to detect.
Also, the changes that you mention are not sudden.  They're normally measured over years.  Lots of other things change in that period of time as well (like runway configurations, changes in obstructions, etc.)  So pilots are already used to having to update their information (like charts) periodically.  When necessary, the magnetic field information is updated on them.   This does mean that the name of a runway might differ from its magnetic orientation over time until it is renamed.
And of course, navigation by non-magnetic means (radio signals, GPS, etc.) are not affected.

I don't think magnetic shifting affects the pilots directly, but it does force airports to re-number their runways because of the fact that runways derive their numbers from their magnetic headings.
Other than that, I don't know of any direct effect this would have on the landing itself, as approaches rely on the ILS's radio signals or direct visual contact with the runway.